After massive experiment, results favor wildlife corridors
To combat urban sprawl and protect wildlife, many communities have set aside land for wildlife corridors linking natural areas to one another.
Public support for these greenways, however, has overshadowed a long-running debate among ecologists about whether they actually achieve their presumed benefits. The debate has been hobbled by a lack of definitive data, with many studies based solely on observations and others only on small-scale experiments, scientists say.
A University of Florida-led study may help resolve the issue. Set to appear next week in the Proceedings of the National Academy of Sciences, the study examines two barometers of healthy ecosystems – plant pollination by insects and the dispersal of seeds by birds – and concludes that corridors encourage the movement of plants and animals across "fragmented" landscape. The findings of the study are important, its authors say, because it is based on a much larger and more ambitious experiment than typically attempted.
"This is by far the largest experimental look at the effects of corridors that has ever been done," said Josh Tewksbury, a UF postdoctoral associate and the studys lead author.
Intuitively, wildlife corridors make sense. First envisioned as early as the 1960s, they are seen as ways to allow wildlife and plants to spread across natural landscapes that have been cut into pieces by roads, development, logging or other disturbances. The idea is that corridors not only allow animals to find new resources, they also prevent the isolation of species – isolation that can lead to localized extinction if the habitat fragments are not accessible for reproduction or recolonization. Finding support for this seemingly simple theory, however, is more difficult than might appear, said Doug Levey, a UF professor of zoology and one of the studys authors.
Previous studies have shown that wild areas connected by corridors have more wildlife or greater biodiversity than disconnected areas. But these studies often failed to account for other factors that may have influenced the observed differences, Levey said. For example, corridors in urban areas often lie along streams or rivers because these flood-prone areas tend to be left undeveloped. But waterways represent one type of habitat that may benefit wildlife and plants more than the corridors themselves, he said. If thats the case, it would be better to preserve such habitats wherever they are rather than only those that connect fragments, he said.
Experiments on corridors, meanwhile, are difficult to pull off because the areas needed to test and repeat them are so large – at least for large animals that typically range over wide areas.
An experiment exploring whether corridors benefit black bears, for example, would require an area equal to the bears range of hundreds of miles. And other similarly huge natural areas would be needed to repeat the experiment to test its conclusions. As a result, most scientifically rigorous corridor experiments have taken place on much smaller scales. One noted experiment, for instance, focused on insect distribution on 20-by-20-square-inch plots of moss arranged in connected and unconnected patches.
The UF-led team sought to increase the scale considerably.
The researchers mapped out eight similar sites, each about 158 acres along the South Carolina-Georgia state line. This site, the Savannah River Site National Environmental Research Park, is a 482-square-mile federally protected research area originally set aside during the Cold War for nuclear weapons development.
Forests of 50-year-old pine trees dominate all eight sites. At the researchers request, the U.S. Forest Service arranged for workers to log trees and burn the remaining groundcover in selected areas, creating one central clearing and four peripheral clearings on each site. They also logged corridors connecting each central clearing to one of the peripheral clearings, leaving the others separated by the forests. Each the clearing measured about two acres.
With 40 total clearings, the scale of the experiment can best be appreciated from the air. As seen from a plane flying at several thousand feet, each one of the eight sites looks more like a large crop circle or alien launch pad than a biology experiment. From a satellite, the arrangement of eight total sites is even more impressive.
Created in 1999, the clearings quickly grew into fields. These habitat "patches" provided what Levey called "black and white" habitat types when compared with the forest – plants and animals found in the fields would never flourish in the forest and vice versa. Research on the sites lasted for two years, with most data collected in 2000 and 2001.
For one of two major experiments, the researchers planted male holly bushes in the central patch and female hollies in the four peripheral patches. They chose holly because it is not naturally present in the forest and the female trees cannot bear fruit unless pollinated by males. The researchers waited until the hollies had flowered and then measured the fruit set, or the percentage of flowers that turned into berries, in each of the clearings.
The result: The hollies in the connected patches were consistently more fruitful than in the unconnected ones. This indicated that more wasps, butterflies and other insect pollinators made it from each central patch through the corridor than through the forest.
When birds or other animals eat fruits, they often distribute the seeds to new locations in their droppings, an important mechanism of plant dispersal. To gauge the effects the corridors had on this process, the researchers marked thousands of seeds of wax myrtle and holly in the central patch with a sticky powder that can be seen only with a florescent light. The researchers then placed seed traps under 16 bird perches built in each of the connected and unconnected peripheral patches. Over several months, they collected and analyzed the resulting bird droppings in a lab.
Given the grand scale of the experiment, the work was not without difficulties. "We collected thousands and thousands of defecations from birds, and it takes a lot of time to go through them all," Tewksbury said. The resulting data revealed that significantly more droppings containing wax myrtle and holly seeds were carried from the central patches to the connected patches than to the unconnected patches. This indicated that more birds were flying between the connected patches than the unconnected ones. "There was almost double the (center patchs) droppings in the connected receiver patches versus the unconnected patches," Levey said.
The findings may go well beyond pollination and seed dispersal. When plants have more pollen, they produce more fruit, attracting more birds, which distribute more seeds, which attract more birds and seed-eating animals, and so on. So although the experiment tested only two processes, it suggests that corridors can be beneficial in the much larger biological community.
"Our study suggests that these corridors do help in connecting populations, and theoretically they should help sustain networks of populations existing in increasingly fragmented landscapes," Tewksbury said.
The research team also included zoologists Nick Haddad, of North Carolina State University, Brent Danielson of Iowa State University, Sarah Sargent of Allegheny College in Pennsylvania and numerous graduate students.
Josh Tewksbury | EurekAlert!